A conventional telecommunication system is illustrated on FIG. 1. In such a system, the network is structured with a plurality of cells. In each cell, e.g. in cell 101 in FIG. 1, a primary station 100 serves the cell. This means that the primary station organizes and controls all communications with secondary stations 110 within the cell. The communication signals are transmitted on different channels. Typically, at least a downlink (from the primary station to the secondary station) data channel, and a downlink control channel are transmitted by the primary station. Similarly, uplink (from the secondary station to the primary station) corresponding data channels and control channels are also transmitted, but for the sake of clarity of the drawing, these channels are omitted from FIG. 1.
In an LTE system for example, the primary station 100 comprises a plurality of antennas and is able to adjust the respective transmit antenna gain and phase to create beamformied data streams towards a secondary station 110. These antenna gains and phases may constitute a precoding matrix (or vector if there is only one transmission beam). A control signalling message on the Physical Downlink Control Channel (PDCCH) is used to signal allocations of transmission resources on user channel PDDCH. In general, a beamformed transmission can be considered to be transmitted via a spatial channel. The reception of a beamformed data stream typically requires a phase and possibly an amplitude reference at the receiver. Such a reference can be provided by transmitting known reference symbols with the same beamforming vector as applied to the data. These reference symbols may be multiplexed with data or control information using known techniques such as TDM FDM or CDM. Therefore a spatial channel may be defined in terms of the reference symbols transmitted using a combination of a set of TDM, CDM, FDM and beamforming vector.
In order that the primary station 100 can schedule downlink data transmissions to make efficient use of system resources, secondary station 110 is typically expected to provide the primary station with feedback on the downlink channel state, for example with explicit feedback like a channel transfer function, an interference power level or an interference covariance matrix. It is also possible to have an implicit feedback, under the assumption of a particular transmission scheme, like a preferred transmission rank, a preferred precoding matrix or vector(s), or an achievable data rate (e.g. CQI). Such feedback is typically based on observation of periodically transmitted reference symbols designed for this purpose (i.e. CSI-RS or Channel State Indicator-Reference Symbol), and estimates of interference.
However, in a system like LTE-Advanced it is desirable to reduce the overhead due to transmission of CSI-RS and therefore CSI-RS are intended to be transmitted infrequently.